1. Field of the Invention
The present invention relates generally to silicon and silicon germanium based semiconductor transistor devices, and more specifically, to a device design including a grown epitaxial field effect transistor structure capable of ultra high-speed, low-noise for a variety of communication applications including RF, microwave, sub-millimeter-wave and millimeter-wave. Preferably, the epitaxial field effect transistor structure includes the critical device scaling and layer structure design for a high mobility strained n-channel transistor incorporating silicon and silicon germanium layers to form the optimum modulation-doped heterostructure on an ultra thin SOI or SGOI substrate in order to achieve fmax in excess of 200 GHz.
2. Description of the Prior Art
The attractiveness of substantial electron mobility enhancement (i.e. 3–5 times over bulk silicon) in modulation-doped tensile-strained Si quantum wells has inspired a long history of device development on Si/SiGe n-channel modulation doped filed-effect transistors (MODFETs). Subsequently, it has been demonstrated that SiGe MODFETs consume lower power and have lower noise characteristics compared to SiGe Heterojunction Bipolar Transistors (HBTs). Similarly, when compared to RF bulk Si CMOS device, SiGe MODFETs still have lower noise characteristics, and higher maximum oscillation frequency (fmax). Consequently, Si/SiGe MODFETs are becoming more and more attractive devices for high speed, low noise, and low power communication applications, where low cost and compatibility with CMOS logic technology are required and often essential. Recently, n-channel MODFETs with long channel lengths ranging from 0.2 μm to 0.5 μm have demonstrated encouraging device performances.
Typically, a Si/SiGe MODFET device have an undoped, tensile strained silicon (nFET) or a compressively strained SiGe (pFET) quantum well channels whereby the induced strain is used to increase the carrier mobility in the channel, in addition to providing carrier confinement. The synergistic addition of modulation doping further improves the carrier mobility in the channel by reducing the ionized impurity scattering from the dopants and further reducing the surface roughness scattering in a buried channel. Record high room temperature mobilities of 2800 cm2/Vs have been achieved for electron mobilities in a tensile strained silicon channel grown on a relaxed Si0.7Ge0.3 buffer. Conversely, very high hole mobility of 1750 cm2/V-s in a pure Ge channel grown on a Si0.35Ge0.65 buffer has been achieved [R. Hammond, et al, DRC, 1999]. The highest fT that has been achieved for a strained silicon nMODFET is 90 GHz [M. Zeuner, 2002], and the highest fmax is 190 GHz [Koester, et al to be published]. So far, neither fT nor fmax has reached 200 GHz with Si/SiGe MODFETs.
As described in a simulation study conducted by the inventors, in order to achieve higher speed, the MODFET has to be scaled properly, both in the vertical dimensions and the horizontal (or lateral) dimensions. However, it turns out that the scaling of MODFETs is even more challenging than for CMOS scaling due to the following: 1) the horizontal scaling brings the source and drain closer, and, like the case in the CMOS, short-channel effects and bulk punchthrough become the major hurdles preventing the lateral scaling; and, 2) the vertical scaling of the layer structure turns out to be crucial. The lateral scaling alone cannot keep the scaling of the performance. However, the vertical scaling of the MODFET structures to reduce the depth of the quantum well (dQW) is quite challenging, particularly due to the scaling and abruptness of the n+ supply layer, which is typically doped with Phosphorus as explained in the Annual Review of Materials Science, vol. 30, 2000, pp. 348–355. FIG. 6 illustrates a graph 200 of the Phosphorus (P) doping profile for a G1 (generation) layer structure and the steady-state P doping 201 problem and transient P doping problems 202 associated with the Phosphorus doping in a CVD growth system.
It would be highly desirable to provide a scaling technique for MODFET device structures that overcomes the lateral and vertical scaling challenges in the manufacture of MODFET device structures.
It has been father been demonstrated in commonly-owned, co-pending U.S. patent application Ser. No. 10/389,145 entitled “Dual Strain State SiGe Layers for Microelectronics” by J. Chu, et al, filed Mar. 15, 2003, the contents and disclosure of which is incorporated by reference as if fully set forth herein, that MODFETs on a thick Silicon-Germanium-on-Insulator (SGOI) substrate will behave like MODFETs on a bulk substrate. Co-pending U.S. patent application Ser. No. 10/389,145 particularly describes a generic MODFET layer structure on a SGOI substrate without specifying the critical layer structure for high performance.
It would be further highly desirable to provide a scaled MODFET device structure that is built on an ultra-thin SiGe-on-insulator (SGOI) substrate, wherein the MODFET device structure exhibits ultra-high speed device performance (e.g., fT, fmax>300 GHz) with better noise figures, acceptable voltage gain and good turn-off characteristics.